The duty of internal heat exchange can be flexibly adjusted without impairing energy saving performance of a HIDiC. A method of adjusting the duty of heat exchange in a heat exchange structure of a HIDiC includes totally condensing a portion of the vapor fed to a heat exchange structure in a heat exchange structure; and providing a liquid control valve downstream of the heat exchange structure on the first line, without providing a control valve on a vapor-flowing part of first and second lines of the HIDiC, and adjusting a flow rate of a portion of the compressor outlet vapor flowing into the heat exchange structure by using the control valve, while compensating for a pressure loss needed for the control valve by using a liquid head of a condensate, and/or by using pressurization by a pump.